Chapter 3: Transport Layer Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion control Transport Layer

Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport Layer

Transport services and protocols provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDP application transport network data link physical network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical application transport network data link physical Transport Layer

Transport vs. network layer network layer: logical communication between hosts transport layer: logical communication between processes Sport:8050 Dport: 25 C A D B Sport:4625 Dport: 80 Transport Layer

Internet transport-layer protocols reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP services not available: delay guarantees bandwidth guarantees application transport network data link physical network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical application transport network data link physical Transport Layer

Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport Layer

Multiplexing/demultiplexing Multiplexing at send host: Demultiplexing at rcv host: delivering received segments to correct socket gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) = socket = process application transport network link physical P1 P2 P3 P4 host 1 host 2 host 3 Transport Layer

How demultiplexing works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format Transport Layer

Connectionless demultiplexing (UDP) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number IP datagrams with different source IP/port can be directed to same socket Create a socket binding to a port number UDP socket identified by two-tuple: (dest IP address, dest port number) Transport Layer

Connectionless demux (cont) P2 client IP: A P1 P1 P3 SP: 6428 DP: 9157 SP: 6428 DP: 5775 SP: 9157 SP: 5775 DP: 6428 DP: 6428 Client IP:B server IP: C Port: 6428 Socket tuple: (dest IP address, dest port number) Two clients’ traffic can be mixed together at server Transport Layer

Connection-oriented demux (TCP) TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket Two connections cannot mixed together at the receiver host Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple Web servers have different sockets for each connecting client Remember the fork() and new socket generated by accept() Transport Layer

Connection-oriented demux (cont) P1 client IP: A P4 P5 P6 P2 P1 P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 SP: 9157 DP: 80 DP: 80 Client IP:B server IP: C S-IP: A S-IP: B D-IP:C D-IP:C Transport Layer

Connection-oriented demux: Threaded Web Server Fork() P1 client IP: A P4 P2 P1 P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 SP: 9157 DP: 80 DP: 80 Client IP:B server IP: C Port: 80 S-IP: A S-IP: B D-IP:C D-IP:C Transport Layer

Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport Layer

UDP: User Datagram Protocol [RFC 768] “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired UDP worm (Slammer) Transport Layer

UDP-based Worm: Slammer Worm code flow: Exploit code (buffer overflow) Generate random target IP address x: Send() worm code to x on udp port 1434 Bandwidth-limited worm Severely congested Internet Stopped ATM, Flight checking, … Fast spreading worm code (Jan. 2003) Single UDP packet: 376 bytes Average scan rate: 4000 scans/sec Infect 90% in 10 minutes ~ 100,000 infected in an hour TCP-based worm is much slower TCP connection setup Connect() is a blocking call Multiple threads for spreading Transport Layer

UDP: more other UDP uses often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! 32 bits source port # dest port # Length, in bytes of UDP segment, including header length checksum Application data (message) UDP segment format Transport Layer

UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Sender: treat segment contents as sequence of 16-bit integers checksum: 1’s complement of addition of segment contents sender puts checksum value into UDP checksum field Receiver: Add all received 16-bit segments, including checksum check if result is 1111 1111 1111 1111: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. Transport Layer

Internet Checksum Example Note When adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 Kurose and Ross forgot to say anything about wrapping the carry and adding it to low order bit wraparound sum checksum Transport Layer

Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport Layer

Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Network layer Transport Layer

Reliable data transfer: getting started rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer deliver_data(): called by rdt to deliver data to upper send side receive side udt_send(): called by rdt, to transfer packet over unreliable channel to receiver rdt_rcv(): called when packet arrives on rcv-side of channel Transport Layer

Reliable data transfer: getting started We’ll: incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify sender, receiver event causing state transition actions taken on state transition state 1 state: when in this “state” next state uniquely determined by next event state 2 event actions Transport Layer

Rdt1.0: reliable transfer over a reliable channel Assumption: underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel Wait for call from above rdt_send(data) Wait for call from below rdt_rcv(packet) extract (packet,data) deliver_data(data) packet = make_pkt(data) udt_send(packet) receiver sender Only need to chop bit-stream data into packets and send Transport Layer

Rdt2.0: channel with bit errors Assumption #1: underlying channel may flip bits in packet checksum to detect bit errors Assumption # 2: no packet will be lost the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): Error detection (checksum) Receiver feedback: control msgs (ACK,NAK) rcvr->sender Sender retransmit if NAK Transport Layer

rdt2.0: FSM specification rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) receiver rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for ACK or NAK Wait for call from above udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) Wait for call from below L sender rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) L : means no action extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

rdt2.0: operation with no errors rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for ACK or NAK Wait for call from above udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) Wait for call from below L rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

rdt2.0: error scenario rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for ACK or NAK Wait for call from above udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) Wait for call from below L rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! Time-out and retransmit can’t just retransmit: possible duplicate Handling duplicates: sender retransmits current pkt if ACK/NAK garbled sender adds sequence number to each pkt receiver discards (doesn’t deliver up) duplicate pkt stop and wait Sender sends one packet, then waits for receiver response Transport Layer

rdt2.1: sender, handles garbled ACK/NAKs rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) Wait for ACK or NAK 0 Wait for call 0 from above udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) L L Wait for ACK or NAK 1 Wait for call 1 from above rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) rdt_send(data) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) udt_send(sndpkt) Transport Layer

rdt2.1: receiver, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) Wait for 0 from below Wait for 1 from below rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) Why ACK for wrong sequence packet? Transport Layer

rdt2.1: discussion Sender: seq # added to pkt two seq. #’s (0,1) will suffice. Why? must check if received ACK/NAK corrupted What if seq. # error? twice as many states state must “remember” whether “current” pkt has 0 or 1 seq. # Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # note: receiver can not know if its last ACK/NAK received OK at sender Transport Layer